ci: add workflow_dispatch trigger to containers.yml

Allows on-demand image builds from any branch via gh workflow run,
producing :sha-<short> tags for downstream integration testing
without widening the push-trigger branches list.

Author: RembrandtK

.github/workflows/containers.yml

@@ -1,6 +1,7 @@
 name: Build and upload Docker image
 
 on:
+  workflow_dispatch:
   push:
     branches:
       - main